1) Field of the Invention
The present invention relates to a method for roasting granular material, such as nuts and coffee beans, and an apparatus therefor. According to the invention, the granular material can be roasted evenly and in a short time.
2) Prior Art
When roasting granular material, such as coffee beans, it is important to heat the material up to a desired temperature speedily and in an even manner in order to obtain a highly rated taste and flavor. Hitherto, a drum type roaster or a jet stream type roaster is well known for roasting such granular material. In the drum type roaster, the material is put in a drum which lays down on a heat source in a horizontal direction and the material is rotated in the drum to be roasted; while in the jet stream type roaster, the material is roasted in a funnel-shaped container into which hot air is introduced from the bottom thereof so that the granular material is gyrated in an up and down direction to be roasted.
The drum type roaster has the advantage that the material can be heated up in an even manner because the material is rotated in the container. However, since the heat source is provided beneath the drum-shaped container that lies down in a horizontal direction and the container is rotated to agitate the material, the driving apparatus for rotating the container becomes large. Further, the heat cannot be effectively transferred between the granular material and the heat source so that it takes a long time to roast the material. Furthermore, according to the drum type roaster, it is necessary to incline the drum container in order to pick the material up after roasting. Therefore, another apparatus for inclining the drum container is required in addition to the apparatus for rotating the container. It takes power and time to incline such a big and heavy drum container, and the material is pyrolized during when the roasted material is picked up from the container. Then, the taste and flavor of the material becomes deteriorated.
On the other hand, in the jet stream type roaster, where the material is put in an upright type funnel-shaped container and roasted by hot air introduced through an opening provided at the lower end of the container, has the problem that the heating effect is poor and thus the roasting speed is slow. Because, in such type of roaster, when the material is blown from the bottom to up, the hot air strongly blows against the material, while, when the material goes down, the hot air hardly catches the material. In addition, in the jet stream type roaster, each piece of the material (beans) is floated in a random manner in the container and therefore the material cannot be roasted evenly.
There is another type of roaster where hot air is blown against the material from an upper side. However, in such an apparatus, the material flies in a container if a necessary amount of air to heat up the material is blown rapidly, and the floating material does not catch the hot air well. Therefore, the heating effect is also poor and the material cannot, be roasted evenly.
Roasting is to heat a material up to a temperature at which a chemical reaction begins in that the material is changed into a desired pyrolized condition; the chemical reaction is caused during the last period of a heating cycle of the material to cause the chemical reaction. In case of coffee beans, for example, the taste of coffee liquid extracted from roasted beans depends remarkably upon the time period until the temperature goes up to the last step. Therefore, the time period of heating up is very important for roasting coffee beans. In accordance with the recent study, it is recognized that if coffee beans are heated up to a desired temperature in a time as short as possible, an expansion rate of the beans becomes highest and that more amount of liquid coffee can be obtained from such well-expanded beans. In addition, it is known that the taste of the liquid coffee extracted from beans which have been roasted in a short, time is more delicious in comparison to liquid coffee extracted from beans which is roasted taking a longer time. However, if the roasting temperature becomes higher than 240.degree. C. essential oil goes out from the beans and then the taste of the coffee liquid becomes bad. Therefore, in order to obtain a highly qualified taste and flavor, it is necessary to make such that the beans catches hot air as much as possible. That is to say, it is required to evenly blow hot air against the beans in an even manner with a high speed.